Method of crosslinking polysilazane polymers

ABSTRACT

The present invention describes a novel method for crosslinking polysilazane polymers having Si-H or N-H bonds. The method comprises mixing the polysilazane with a silazane crosslinker having at least 2 boron functional groups which can react with the Si-H or N-H bonds of the polysilazane and then facilitating crosslinking.

BACKGROUND OF THE INVENTION

The present invention relates to a method of crosslinking polysilazanepolymers in which boron modified silazanes are used as crosslinkers. Theinvention also relates to a novel crosslinker comprising boron modifiedtris(trimethylsilylamino)silane and a method for it manufacture.

A variety of polysilazane oligomers, cyclics, resins and linear polymersare known in the art. Such polysilazanes are characterized as havingbackbones with alternating silicon and nitrogen atoms. These polymershave found broad utility as precursors to a variety of ceramic materialssuch as ceramic monoliths, ceramic fibers and matrices for ceramicmatrix composites.

To be truly useful as ceramic precursors, however, the polysilazanesmust be curable (infusible) to prevent deformation of the ceramic uponheating. Various approaches to providing curability have been suggested.For instance, Mahone in U.S. Pat. No. 5,086,126 discloses a process foradding vinyl groups to a polysilazane such that upon addition of a freeradical precursor the polymer would rapidly cure.

Similarly, various references disclose the addition of boron compoundsto polysilazanes to provide curability. For instance, Zank in U.S. Pat.No. 5,169,908 discloses the addition of borane to a hydridopolysilazanepolymer to render the polymer curable. Funayama et al. in U.S. Pat. No.5,030,744 discloses the addition of a boron compound to a polysilazaneto increase its molecular weight. U.S. Pat. No. 4,910,173 granted toNiebylski discloses the formation of an organoborosilazane by thereaction of a boroxine with a polysilazane. Seyferth et al. in J. Am.Ceram. Soc. 73, 2131-2133 (1990) teaches the reaction of a silazaneoligomer with borane to form a higher molecular weight borazine.Finally, Noth in Z. Naturforsch, B. Anorg. Chem. Org. Chem. 16 [9]618-621 (1961) teaches the reaction of hexamethyldisilazane withdiborane to form a higher molecular weight borazine.

As is readily apparent, each of the above references teaches theaddition of boron to a polysilazane to render it infusible or toincrease the molecular weight of the resultant polymer. By contrast, thepresent inventors have now discovered that boron-modified silazanecrosslinkers can be utilized to render nearly any polysilazane havingSi--H or N--H bonds infusible.

SUMMARY OF THE INVENTION

The present invention relates to a method of crosslinking a polysilazanehaving Si--H or N--H bonds. The method comprises mixing the polysilazanewith a silazane crosslinker which has at least 2 boron functional groupsthat react with the Si--H or N--H bonds. The crosslinking of the mixturemay then be facilitated by heating for a time and at a temperaturesufficient to crosslink the polysilazane to the desired extent.

The present invention also relates to a novel silazane crosslinker ofthe structure: ##STR1##

Finally, the invention relates to a method of forming the novelcrosslinker which comprises reacting tris(trimethylsilylamino)silanewith borane for a time and at a temperature sufficient to form the boronmodified silane.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that a boron modifiedsilazane crosslinker can be used to crosslink polysilazanes and, thus,render them more useful as ceramic precursors. The approach claimed inthe present invention allows for better control over the crosslinkingreaction and can lower the T_(g) and storage modulus of the resin toimpart a controllable tack depending on the level of crosslinkeraddition.

The method of curing the polysilazanes of the present invention involvesmixing the polysilazane with the crosslinker followed by facilitation ofthe crosslinking reaction. Upon initiation, the boron of the crosslinkerreacts with the Si--H and/or N--H bonds of the polysilazane to formSi--B and/or N--B bonds, respectively, and, thus, cause crosslinking.

The polysilazanes which are useful herein can be any which have N--H orSi--H bonds for reaction. Representative non-limiting examples of suchpolymers include those of Gaul in U.S. Pat. Nos. 4,312,970, 4,395,460,and 4,340,619, those of Cannady in U.S. Pat. No. 4,540,803, those ofGerdau et al. in European Patent 351,747, those of U.S. Pat. No.4,543,344, those of European Patent 332,374, those described by Funayamaet al. in U.S. Pat. No. 5,030,744 and those of Lebrun et al. in U.S.Pat. Nos. 4,656,300 and 4,689,252, the disclosures of which are allhereby incorporated by reference.

The preferred polymers to be used herein are those of Cannady in U.S.Pat. No. 4,540,803. These polysilazanes are prepared by a method whichcomprises contacting and reacting in an inert essentially anhydrousatmosphere, trichlorosilane and a disilazane at a temperature in therange of 25° C., to 300° C. while distilling volatile byproducts. Thedisilazane used in the process has the formula (R₃ Si)₂ NH where R isselected from the group consisting of vinyl, hydrogen, phenyl and alkylradicals containing 1 to 3 carbon atoms.

The trichlorosilane is treated with the disilazane in sufficient amountsto react with all of the chlorine in the chlorine containing silane.This is usually an equimolar amount based on the chlorine content of thetrichlorosilane.

The disilazane used in the Cannady invention has the formula (R₃ 'Si)₂NH, where R' is vinyl, hydrogen, an alkyl group of 1-3 carbon atoms or aphenyl group. Thus, the R' groups are independently selected from thegroup consisting of hydrogen, methyl, ethyl, propyl, vinyl and phenyl.Examples of suitable disilazanes include [(CH₃)₃ Si]₂ NH, [(C₆ H₅ (CH₃)₂Si]₂ NH, [(C₆ H₅)₂ CH₃ Si]₂ NH, [CH₂ ═CH(CH₃)₂ Si]₂ NH, [CH₂ ═CH(CH₃)C₆H₅ Si]₂ NH, [CH₂ ═CH(C₆ H₅)₂ Si]₂ NH, [CH₂ ═CH(C₂ H₅)₂ Si]₂ NH, [H(CH₃)₂Si]₂ NH, and [CH₂ ═CH(C₆ H₅)C₂ H₅ Si]₂ NH.

An especially preferred embodiment of the Cannady invention involves thereaction of trichlorosilane with hexamethyldisilazane. The resultantpolymer produced thereby, hydridopolysilazane, has been shown to havevaluable preceramic properties.

The above reactants are brought together in an inert, essentiallyanhydrous atmosphere. By inert it is meant that the reaction is carriedout under a blanket of inert gas such as argon. nitrogen or helium. Whatis meant by essentially anhydrous is that the reaction is preferablycarried out in an absolutely anhydrous atmosphere but minute amounts ofmoisture can be tolerated.

When the reactants are contacted with each other an intermediate aminocompound is formed. It is preferred that the reactants are broughttogether in such a manner to keep the initial reaction exotherm to aminimum. Upon continued heating additional amino compound is formed and,with further heating, R₃ SiCl is distilled from the reaction mixture andthe silazane polymer formed. For best results, the rate of heatingshould be controlled at a rate of less than about 1° C./min. A heatingrate of about 0.5° C./min. or less is preferred. As the temperature ofreaction is raised, more condensation takes place and crosslinkingoccurs with residual R₃ Si that is not distilled from the mixture actingas a chain stopper. This control allows one to stop the reaction at anypoint to obtain almost any desired viscosity. The desired temperaturerange for the reaction is 25° C., to 300° C, with a temperature in therange of 125° C., to 275° C. being more preferred. The length of timethat the reaction requires depends on the temperature employed and theviscosity one wishes to achieve.

Although the polymers of the Cannady invention are specifically setforth, nearly any polysilazane with N--H or Si--H bonds may be used inthe invention.

The crosslinker used in the present invention is a silazane which has atleast 2 boron functional groups. Generally, such silazanes have a lowmolecular weight, e.g., less than about 1000. Although any suchcrosslinker would be functional herein, generally they have thestructure:

    R.sub.3 Si--NR--(SiR.sub.2 --NR).sub.x --SiR.sub.3

In this structure, R is independently a hydrogen, a hydrocarbon or 1-20carbon atoms, a hydrocarbon of 1-20 carbon atoms substituted withsilicon, nitrogen or boron, or a substituted silicon, nitrogen or boronatom. Specific examples include alkyls such as methyl, ethyl, propyl,butyl, etc, alkenyls such as vinyl, aryls such as phenyl, cycloalkylssuch as cyclohexyl, alkaryls, alkylaminos, aminoalkyls, alkylsilyls,silylalkyls, aminosilyls, aminoalkylsilyls, borosilyls, boroaminosilyls,boroalkyls and the like. The above R groups must contain at least 2boron functional groups per crosslinker molecule. These boron functionalgroups can be independently selected from the group consisting ofhydrogens, halogens, alkoxys, or hydroxys attached to boron. x in theabove structure is 0-5.

A preferred crosslinker for use in the present invention has thestructure: ##STR2## Since each of the B--H bonds of the crosslinker arepotential sites for reacting with Si--H or N--H bonds of a polysilazane,the crosslinker is hexafunctional and, thus, can effectivelyinfusibilize the polymer even when used in small amounts.

This crosslinker can be prepared by a number of techniques. Generallyfor convenience, however, it is formed by reactingtris(trimethylsilylamino)silane with borane to produce the desiredcrosslinker. Both reactants are known in the art and commerciallyavailable. Generally, any source of borane may be used. For instance,borane is available from Aldrich Chemical Co. as complexes with variousLewis bases. These include borane complexes with various amines such aspyridine, butylamine or diethylamine, complexes with sulfides such asmethyl sulfide, complexes with phosphines such as triphenylphosphine andcomplexes with ethers such as tetrahydrofuran. Although any source ofborane may be used, the present inventor has found it convenient to usethe borane-tetrahydrofuran complex.

The stoichiometric amount of borane used in this reaction is three molesof borane per mole of tris(trimethylsilylamino)silane. It is generallypreferred to use two to three moles of borane per mole oftris(trimethylsilylamino)silane. However, greater or lesser amounts maybe used. If less than a stoichiometric amount is used, residual N--Hwill be present. If greater than a stoichiometric amount is used, excessborane may be present after the reaction Which may cause other undesiredreactions.

The reaction of the silane can be conducted with or without a solvent.The solvents which may be used herein include any which act as a solventfor the borane, the silane and the boron modified silane withoutadversely affecting any of the species. Examples of such solventsinclude alkanes such as pentane, hexane, heptane, octane etc, etherssuch as tetrahydrofuran, or aromatic hydrocarbons such as benzene,toluene, xylene etc. Generally, if the borane-tetrahydrofuran complex isused in the reaction it is convenient to use tetrahydrofuran or mixtureswith aromatic hydrocarbons as the solvent.

The reaction of the silane and borane is conducted by mixing the silaneand the borane in a suitable reaction vessel. This reaction can beperformed at any suitable temperature or pressure and in any convenientatmosphere. For simplicity, however, it is generally run at roomtemperature under an inert atmosphere and at atmospheric pressure. Sincean exotherm generally occurs when the silane and the borane are mixed,it is often preferred to control the exotherm by slowly adding theborane to a solution of the silane. Continued stirring of this mixture(e.g., for 1-24 hours) results in formation of the desired modifiedsilane.

The boron modified silane produced by the above reaction is then merelyrecovered from solution. Numerous methods such as simple evaporation orstripping of the solvent under heat and/or vacuum are known in the artand useful herein.

Although one specific crosslinker is detailed above, other boronmodified silazanes would also function herein. For instance, boronmodified hexamethyldisilazane and materials of the structure R₃ SiNBH₂SiR₂ NBH₂ SiR₃ are useful herein, Such other crosslinkers are eitherknown in the art or can be prepared using methods known in the art.

The above crosslinkers are then mixed with the polysilazane and thecrosslinking reaction is initiated. The polysilazane and the crosslinkermay be mixed together in their liquid state or, alternatively, they maybe blended in a solvent. The solvents which may be used herein includeany which act as a solvent for both the polysilazane and the crosslinkerand which do not cause rearrangement of either species. Examples of suchsolvents include alkanes such as pentane, hexane, heptane, octane etc.,ethers such as tetrahydrofuran, or aromatic hydrocarbons such asbenzene, toluene, xylene etc.

The crosslinker and the polysilazane may be blended in nearly any ratiodesired to provide sufficient tack, flow, and final cure. Generally,however, the crosslinker is present in an amount of at least about 0.01wt. % crosslinker based on the weight of the polysilazane with a rangeof about 0.01 to about 50 wt. % being preferred. In addition, it is alsocontemplated herein that several polysilazanes (e.g., of varyingviscosity), several crosslinkers or other desirable materials (e.g.,ceramic fillers) may be blended with the mixture to provide desirableproperties.

The polysilazane/crosslinker mixture is then exposed to conditions whichfacilitate the crosslinking reaction. Generally, this involves merelyheating the mixture to a sufficient temperature. Temperatures in therange of 50°-500° C., are generally sufficient. Other means of inducingcrosslinking such as radiation or crosslinking catalysts are, however,also contemplated.

The polysilazane and crosslinker blend is useful for many purposes suchas in the formation of fibers, monoliths and as matrices for ceramicmatrix composites. In addition, the material may be used to impregnateporous ceramic bodies to increase density.

The following non-limiting examples are provided so that one skilled inthe art may more fully understand the invention. In these examples, ¹ HNMR spectra were recorded on a Varian or EM390 spectrometer. FTIR datawere recorded on a Perkin Elmer Series 1600 spectrometer. Gel permeationchromatography (GPC) data were obtained on a Waters GPC equipped with amodel 600 E systems controller, a model 490 UV and model 410Differential Refractometer detectors: all values are relative topolystyrene. TMA data were recorded on a Du Pont 940 thermomechanicalanalyzer (TMA) interfaced to an Omnitherm 2066 computer.

Carbon, hydrogen and nitrogen analyses were performed on a ControlEquipment Corporation 240-XA Elemental Analyzer. Boron and silicon wasdetermined by a fusion technique which consisted of converting thesilicon material to soluble forms of silicon and analyzing the solutefor total silicon by atomic absorption spectrometry.

All furnace firings were done in an Astro graphite furnace equipped withEurotherm temperature controllers. The furnace was equipped with anIrcon Modeline Plus optical pyrometer to monitor the temperature above900° C.

EXAMPLE 1: Preparation of Boron Modified Tris(trimethylsilylamino)silaneRoute A

A 500 mL 3 necked flask fitted with an argon inlet, an overhead stirrerand an addition funnel was charged with 29.3 gtris(trimethylsilylamino)silane (0.10 mole) distilled from the reactionof trichlorosilane and hexamethyldisilazane under argon. The additionfunnel was charged with 300 mL of a 1.0M BH₃ -THF solution in THF(obtained from Aldrich Chemical Company). This borane solution was addedto the flask over a 2 hour period which was accompanied by a mildexotherm and gas evolution. The resulting solution was stirred 16 hoursand then stripped of volatiles at 60° C. in vacuo resulting in 32 g of aliquid product.

The IR spectrum of tris(trimethylsilylamino)silane was compared with theabove boron modified product. The spectrum of the boron modified productshowed stretches at 2450 cm⁻¹ and 1350 cm⁻¹ indicating the presence ofB--H and N--H bonds respectively. Additionally, stretches at 3350 cm⁻¹indicating the presence of N--H bonds were decreased in the boronmodified product.

Route B

A 1 L 3 necked flask fitted with an argon inlet, an overhead stirrer andan addition funnel was charged with 500 mL of a 1.0M BH₃ -THF solutionin THF (obtained from Aldrich Chemical Company). The addition funnel wascharged with 54 g tris(trimethylsilylamino)silane (0.18 mole) distilledfrom the by-products of the reaction of trichlorosilane andhexamethyldisilazane under argon. The silane was added to the flask overa 1 hour period which was accompanied by a mild exotherm and gasevolution. The resulting solution was stirred 48 hours and then strippedof volatiles at 60° C, in vacuo resulting in 57 g of a liquid product.

EXAMPLE 2: Cure Properties

Hydridopolysilazane made by the method of Cannady in U.S. Pat. No.4,540,803 was blended with the boron modifiedtris(trimethylsilylamino)silane of Example 1 at 30 parts per weightcrosslinker per 100 parts hydridopolysilazane in a toluene solution. Asa control, tris(trimethylsilylamino)silane was also blended with thehydridopolysilazane in the same amounts. These solutions were then usedto impregnate fiber braids suitable for torsional braid analysis (TBA).The braids were dried, placed in a TBA spectrometer for testing andheated to 288° C, for 6 hours. The storage modulus is equivalent to thestiffness of the sample and shows that the sample containing the boronmodified crosslinker increases in stiffness corresponding to thecrosslinking reaction. The fact that little increase in stiffness isseen with the control implies that very little crosslinking is occurringin that system

EXAMPLE 3: Char Composition

Hydridopolysilazane made by the method of Cannady in U.S. Pat. No.4,540,803 (Mw=10,000) was blended with the boron modifiedtris(trimethylsilylamino)silane of Example 1 at varying ratios. Theblends were fired at 1400° C., to determine the effect of thecrosslinker on the composition of the final ceramic. The results arepresented in the following table. It should be noted that thecrosslinker does not have a significant impact on the final ceramicchar, especially the carbon content which could be detrimental to thecomposite oxidation properties.

    ______________________________________                                        Parts Crosslinker/                                                            100 parts HPZ                                                                              Char Yield    % C    % N                                         ______________________________________                                        --           69.60         11.82   28.17                                      10           70.08         12.40   26.54                                      20           70.19         12.49   25.76                                      30           71.11         12.32   24.60                                      ______________________________________                                    

That which is claimed is:
 1. A silazane crosslinker of the structure:##STR3##
 2. A method of forming a crosslinker of the structure: ##STR4##comprising reacting tris(trimethylsilylamino)silane with borane for atime and a temperature sufficient to form the crosslinker.
 3. The methodof claim 2 wherein the amount of borane is about three moles of boraneper mole of tris(trimethylsilylamino)silane.
 4. The method of claim 2wherein the reaction is run at room temperature under an inertatmosphere and at atmospheric pressure.
 5. The method of claim 2 whereinthe reaction is conducted in a solvent.